Lossless optical signal splitter including remotely pumped amplifier

ABSTRACT

A lossless optical component includes an input and at least one output. The optical component includes an operational portion and an amplifier portion upstream of the operational portion. The amplifier portion includes an optical amplifier for amplifying optical signals received by the input. The optical amplifier is provided with input optical waveguide via which the optical amplifier is optically pumpable by a remote pump laser. The output of the optical amplifier is proportional to the loss of the operating portion of the optical component.

This invention relates to an optical coupler for incorporation in anoptical fibre communications network.

Throughout this specification, the term "optical" is intended to referto that part of the electromagnetic spectrum which is generally known asthe visible region, together with those parts of the infra red and ultraviolet regions which are capable of being transmitted by dielectricwaveguides such as optical fibres.

An optical fibre communications network is used to distributeinformation (optical signals) from one or more transmitting stations toone or more receiving stations. For telecommunications purposes, passiveoptical networks, such as TPON (telephone by passive optical networks),are advantageous in that they permit telecommunications over a networkusing a single transmitter (a laser located at an exchange connected tothe network). The main advantage of TPON is that no electric componentsare required in the field. A disadvantage of TPON is that it requiresthe use of optical splitters to pass optical signals from a transmitter(exchange) to a plurality of receiving stations (customers' telephones).TPON is, therefore, limited by the loss at the splitters (typically aTPON system will service only 32 customers per laser). One way toincrease this ratio would be to incorporate optical amplifiers into thesystem. This could be achieved by amplifying the optical signals bymeans of optical amplifiers at one or more positions along the network,for example by using a power amplifier at the transmitter, repeateramplifiers along the network paths, or pre-amplifiers at the receivingstations. In this connection, it should be noted that safetyconsiderations limit the maximum power which can be delivered by thehead end (exchange) laser.

A known type of optical amplifier employs an electric regenerator forboosting power to compensate for splitter losses. The disadvantages ofelectric regenerators are that they are expensive, directional and arenot data transparent. Another known type of optical amplifier (thesemiconductor laser amplifier) overcomes some of the disadvantages ofusing electric regenerators, in that a semiconductor laser amplifier isbi-directional and data transparent. Unfortunately, however, asemiconductor laser amplifier requires an electrical power source, andthis detracts from the main advantage of TPON, namely having onlypassive components in the field.

The present invention provides an optical coupler having an input and aplurality of outputs, the optical component comprising a splitterportion and an amplifier portion upstream of the splitter portion,wherein the amplifier portion includes an optical amplifier foramplifying optical signals received by the input, the optical amplifierbeing provided with input optical wave guiding means via which theoptical amplifier is optically pumpable by a remote pump laser, andwherein the optical amplifier has a gain which is at least equal to theloss of the splitter portion.

In a preferred embodiment, the optical amplifier is a doped fibreamplifier constituted by a length of Er³⁺ doped fibre. Preferably, theinput optical waveguiding means is connected to the doped fibreamplifier via a first WDM, and the first WDM is upstream of the dopedfibre amplifier. In this case, the input may be connected to the firstWDM, the doped fibre amplifier may be connected to the output via asecond WDM, and the component may further comprise a filter downstreamof the second WDM.

The invention also provides an optical system comprising an opticalsource, an optical coupler and a pump laser, the optical coupler beingas defined above, the optical source being connected to the input of theoptical coupler, and the pump laser being connected to the input opticalwave guiding means.

Advantageously, the system further comprises an agc unit, the agc unitand pump laser being connected to the input optical wave guiding meansby means of a further WDM. In the case where the input is connected tothe first WDM, the input optical wave guiding means may be connected tothe first WDM via another WDM, the downstream end of the doped fibreamplifier may be connected to said another WDM via a coupler.Preferably, the coupler is a 10/90 coupler which directs 10% of theoutput of the doped fibre amplifier to said another WDM. Alternatively,where the first WDM is downstream of the doped fibre amplifier, theinput may be connected directly to the upstream end of the doped fibreamplifier.

Advantageously, the optical source is a laser which emits light at 1536nm, in which case the optical amplifier is arranged to have its maximumamplification at this wavelength. Alternatively, the optical source maybe constituted by first and second lasers which are connected to theinput by an input WDM and an optical wave guide. Preferably, the firstlaser emits light at 1300 nm, and the second laser emits light at 1536nm. In this case, the optical amplifier is arranged to have its maximumamplification at a wavelength of 1536 nm, the optical amplifier beingtransparent at 1300 nm. Conveniently, means are provided for modulatinga plurality of radio carrier signals with video signals, and means areprovided for mixing the modulated radio carriers, the resulting analoguesignal being used to modulate the second laser. The pump laser may emitlight at 1480 nm.

Two forms of optical transmission system, each of which incorporates alossless coupler constructed in accordance with the invention, will nowbe described in detail, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic representation of the first form of opticaltransmission system;

FIG. 2 is a schematic representation of the amplifier unit which formspart of the system of FIG. 1;

FIG. 3 is a schematic representation showing a modified form of part ofthe system of FIG. 1;

FIG. 4 is a schematic representation showing another modified form ofpart of the system of FIG. 1; and

FIG. 5 is a schematic representation of the second form of opticaltransmission system.

Referring to the drawings, FIG. 1 shows a passive optical network (TPON)system having a signal laser 1 connected to a lossless coupler 2 by anoptical fibre 3 which defines a 2 km signal path 4 (shownschematically). The laser 1 is a distributed feedback (DFB) laser whichemits light at 1536 nm. The lossless coupler 2 includes an amplifierunit 2a and 4-way splitter 2b. Each of the outputs of the splitter 2bleads to a respective 8-way splitter 5 (only one of which is shown). Thesystem is such, therefore, the signals from the laser 1 can betransmitted to 32 receiving stations (not shown) associated with theoutputs of the splitters 5. The amplifier unit 2a is arranged to providesufficient amplification to signals arriving along the signal path 4 tocompensate for the loss associated with the splitter 2b. This ensuresthat the power budget of the system is adequate to power the 32receiving stations.

The amplifier unit 2a (see FIG. 2) includes a doped fibre amplifier 6constituted by a length of Er³⁺ doped fibre. The amplifier 6 is pumpedoptically by a high power pump laser 7, via a dedicated optical fibre 8.The laser 7 is a 40 mW, 1480 nm laser, though a higher power laser couldbe used with advantage. Because of the high power of the pump laser 7,the optical fibre 8 needs to be protected and armoured to protectpersonnel from the high light levels carried thereby. The optical fibre8 (termed the optical main) is, therefore, analogous to an electricpower cable, and the optical fibre 3 is analagous to an electricalsignal cable. As with electrical connections, the optical signal andpower fibres 3 and 8 are kept separate, and are clearly markedaccordingly.

The amplifier unit 2a also includes two WDMs 9a and 9b positioned atopposite ends of the fibre amplifier 6. The WDM 9a multiplexes the 1480nm pump and the 1536 nm signal, and inputs the multiplexed light to thefibre amplifier 6. The WDM 9b demultiplexes the light amplified by theamplifier 6, and outputs the demultiplexed light to a ferrule filter 10.The filter 10 is a band pass filter having a narrow pass range of 1530nm to 1540 mm (though this could, with advantage, be narrower), and sois effective to filter out any excess light from the pump laser 7. Thefilter 10 also removes noise, that is to say any unwanted spontaneousemissions from the doped fibre amplifier 6.

The amplifier 6 has a gain of 6 dB which is just sufficient tocompensate for the loss in the splitter 2b. Consequently, the coupler 2is essentially a lossless coupler. This lossless coupler 2 has a numberof important advantages, namely:

(i) It utilises an optical amplifier 6 that amplifies the signaldirectly without recourse to electronics.

(ii) The amplifier 6 is pumped optically from a remote position, sothere is no need for a separate power supply for the amplifier in thecabinet which houses the coupler 2.

(iii) By removing the loss associated with the first splitter 2b, powerlevels are maintained fairly constant throughout the system, and thisleads to a safer system which is easier to maintain. It also facilitatesthe location of faults.

(iv) The increased power available downstream of the splitter 2bfacilitates extension of the system. Thus, the system could support agreater splitting ratio, so that up to 128 customers could be servicedby a single laser. The increased power also permits the use of cheapercomponents (such as low power lasers and cheaper receivers), therebymaking the system more cost-effective.

(v) It is bi-directional, and can be used with both digital and analoguesystems.

The amplifier unit 2a is a co-propagating amplifier, that is to say thepump power passes along the fibre amplifier 6 in the same direction asthe signal. The co-propagating amplifier could, however, be replaced bya counter-propagating amplifier, that is to say one in which the pumppasses along the fibre amplifier 6 in the opposite direction to thesignal. A co-propagating amplifier has the advantage of being opticallyquieter than a counter-propagating amplifier, but has the disadvantageof requiring additional optical filtering downstream (in the directionof signal propagation) of the amplifier to remove excess pump power.Conversely, a counter-propagating amplifier has the disadvantage ofbeing relatively optically noisy, but has the advantage of not requiringoptical filtering (except perhaps upstream of the amplifier at, forexample, a head end receiver). A counter-propagating amplifier also hasthe advantage of a higher output power.

Although the coupler 2 described above is inherently lossless wheninitially installed, this may not be the case as the system ages. Thereasons for this are:

(a) Lasers age, reducing their output power with time. Although this isnot normally too much of a problem, this is not the case with a pumplaser. Thus, the gain of the amplifier 2a is exponentially dependantupon the pump power, so that a small change in pump power leads to amuch larger change in the output of the amplifier.

(b) The fibre link 8 to the coupler 2 is sensitive to environmentaleffects. Thus, although a 0.5 dB change in fibre loss is insignificantto a normal system, this deviation in pump power would be serious. Forexample, if the amplifier 2a has a gain of 20 dB, a 0.5 dB decrease inpump power reduces the amplifier gain to about 18 dB.

An obvious solution to these ageing problems is to sample the output byreflecting some of the amplifier output back towards the pump laser.Unfortunately, this is not practical with a fibre amplifier, as thereflection will cause the amplifier to oscillate, that is to say to actas a laser.

FIGS. 3 and 4 show two solutions to the ageing problems, both of thesesolutions relying upon automatic gain control (agc) to stabilise theoutput power of the amplifier. Thus, FIG. 3 is a schematicrepresentation of that part of a TPON system which is equivalent to thesystem of FIG. 1 from its head end to its amplifier unit. FIG. 3 shows asignal laser 11 connected to an amplifier unit 12a by an optical fibre13. The amplifier unit 12a includes a doped fibre amplifier 16constituted by a length of ER³⁺ doped fibre. The amplifier 16 is pumpedoptically by a high power pump laser 17, via a dedicated optical fibre18. A WDM 19a upstream of the amplifier 16 connects the amplifier to thefibre 13 and 18, an additional WDM 19c being positioned in the fibre 18leading to the WDM 19a. The pump laser 17 and an agc unit 20 areconnected to the fibre 18 by means of a further WDM 19d. A 90/10 coupler21 downstream of the amplifier 16 feeds 10% of the amplifier's output tothe WDM 19c.

The arrangement shown in FIG. 3 operates in the following manner. Aswith the embodiment of FIGS. 1 and 2, pump power travels to theamplifier 16 separately from the signal. Pump power travels through theWDMs 19c and 19a to reach the amplifier 16, and 10% of the amplifier'soutput (the returned signal) is fed back to the fibre 18 via the WDM19c. The WDM 19d separates the returned signal from the outgoing pumplaser signal and feeds it to the agc unit 20. If this unit 20 detects adrop in the returned signal (which is proportional to a drop in theamplified signal leaving the amplifier unit 12a), it increases theoutput of the pump laser 17 to compensate for the fall in the output ofthe amplifier 16. In this way, the output of the amplifier unit 12a isstabilised. Apart from this, the main advantage of this arrangement isthat it is very stable, and so is usable with both co-propagating andcounter-propagating amplifiers. One possible disadvantage, which may beimportant in some configurations, is its component count and hence itscost. Also, pump power has to pass through three WDMs, and so willsuffer extra loss before it reaches the amplifier unit 12a.

FIG. 4 shows an alternative agc stabilised arrangement which has a lowercomponent count. This arrangement is similar to that shown in FIG. 3, solike reference numerals will be used for like parts, and only the partswhich are different will be described in detail. Thus, the amplifier 16of the FIG. 4 arrangement is a counter-propagating amplifier, so the WDM19a is positioned downstream of the amplifier. This arrangement relieson the inherent imperfections of WDMs which allows a small amount of theoutput signal of the amplifier 16 to "leak" across the WDM 19a into thefibre 18, and hence back to the agc unit 20 via the WDM 19d. Asmentioned above, the main advantage of this arrangement is its lowcomponent count. A possible disadvantage of the arrangement is itsreliance on the stability of the WDM 19a. If this drifts more thannegligibly, the agc reference signal (that is to say the returnedsignal) will change, thus altering the output of the amplifier 16.

The arrangements of FIGS. 3 and 4 each use an agc unit which relies onan ac agc technique. The reason for using an ac technique is as follows.Generally an agc unit compares the output signal of a component to beregulated with a set reference, and changes the gain of the amplifier tokeep this constant. The simplest method is to detect the mean output ofthe signal, that is to say the `dc` level. Unfortunately, this techniquehas problems when used with a fibre amplifier, because of spontaneousemission and the excess pump light. The agc unit cannot distinguishbetween the signal and these other sources. One option is to use opticalfiltering, but this limits the bandwidth over which the unit can beused.

The ac technique involves adding a small extra amplitude modulation ontop of the normal signal. This will not interfere with the most populartransmission methods (digital or frequency modulation). The agc unit issensitive to signals only at this frequency. Hence, the excess pump andspontaneous emission, which are essentially constant, are ignored. Thisneeds no optical filtering and so the full optical bandwidth of theamplifier can be used.

As the type of lossless coupler described above is bi-directional,systems can be constructed which permit two separate types of signal tobe carried with different power budgets at different frequencies. Asystem of this type will now be described with reference to FIG. 5. FIG.5 shows a passive optical network system having two signal lasers 31aand 31b connected to a lossless coupler 32 by an optical fibre 33 and aWDM 34 which multiplexes the signals from the two lasers onto theoptical fibre. The laser 31a is a Fabry Perot laser which emits light at1300 nm, and the laser 31b is a DFB laser which emits light at 1536 nm.The laser 31a is a standard TPON laser, so that the network can operateas a TPON network (i.e. a 2-way time multiple access 20 Mb/s digitaltelephony system). The laser 31b is used to upgrade the network to BPON(broadband passive optical network), in a manner described below.

The lossless coupler 32 includes an amplifier unit 32a and a 4-waysplitter 32b, these devices being identical to the corresponding partsof the coupler 2 of FIGS. 1 and 2. Thus, the amplifier unit 32a includesa fibre amplifier and a pair of WDMs. The WDMs pass both 1.55 μm and 1.3μm signals, the 1.55 μm signal being amplified whilst the 1.3 μm signalcan pass through the amplifier with little loss. The amplifier is pumpedoptically by a high power (40 mW, 1480 nm) laser 37, via a dedicatedoptical fibre 38. As with the embodiment of FIGS. 1 and 2, each outputof the splitter 32b leads to a respective 8-way splitter 35 (only one ofwhich is shown), so that the system can service 32 receiving stations 39(only one of which is shown) via respective output fibres 40. Eachreceiving station 39 includes a WDM 41 for demultiplexing the 1300 nmand 1536 nm signals carried by the associated fibre 40. The WDM 41 hastwo output fibres 42a and 42b which lead respectively to a telephoneinstrument 43 and a receiver 44. The receiver 44 is a low cost PINreceiver which feeds signals to a down converter 45 to recover BPONsignals.

BPON permits the transmission of many (16 or 32 typically) channels ofvideo on a sub-carrier multiplexed system. In the embodiment shown inFIG. 5, 16 or 32 radio carriers at 950-1750 MHz are modulated, at 46,with video signals. The modulated carriers are then mixed together, andthe resultant analogue signal is used to modulate the laser 31b fortransmission down the optical fibre 33. The amplifier and associatedWDMs are transparent at 1300 nm, so TPON signals are unaffected by thelossless coupler 32. This permits the network to carry both TPON andBPON signals, with both transmitters (the lasers 31a and 31b) beingsituated at the head end (the exchange). This is an improvement overknown BPON systems, which require four lasers to service 32 customers,whereas the system of FIG. 5 requires only one laser per 32 customers.As the lasers needed for BPON cost about 3000, it will be apparent thatthe system of FIG. 3 gives a substantial cost saving. The system couldalso be extended, for example to complement TPON systems in which 128customers are serviced by a single TPON laser, by increasing thesplitting ratio for both TPON and BPON signals. Furthermore, known BPONsystems require the use of expensive avalanche photodiodes (APDs) at thereceiving stations instead of the cheap PINs used in the system of FIG.5. Here again, therefore, the system of the invention leads to asubstantial cost reduction. This system has the additional advantagethat an entire TPON network can be installed with lossless couplersadapted to amplify BPON signals, and this network can be subsequentlyconverted to dual TPON/BPON operation merely by the addition of the BPONtransmission equipment and the pump laser at the exchange.

It would, of course, be possible to modify the system of FIG. 5 by theinclusion of an agc unit in association with the pump laser 37. In thisway, the output of the amplifier unit 32a can be stabilised, even overextended periods of use.

It would also be possible to operate both TPON and BPON at about 1500nm, in which case both types of signal would be amplified at thelossless coupler. Unfortunately, this would require the use of verynarrow channel spacing demultiplexers (one per customer) and this would,at the present time, be prohibitively expensive.

An important advantage of using lossless couplers in opticaltransmission networks, is that they permit the use of any combination ofsimplex, duplex, analogue and digital transmission systems. Moreover,because this type of lossless coupler incorporates an optical amplifier,it does not require conversion to electronics for signal amplification.Consequently, this type of lossless coupler is data transparent, that isto say it permits data to be transmitted at any data transmission rate.This is to be compared with known arrangements which incorporateelectrical amplifiers (regenerators) which operate successfully onlyover a narrow range of data transmission rates.

Although the signal lasers 1, 11 and 31b are stated to emit light at1536 nm, it will be understood that these lasers could emit light atother wavelengths, typically within the range of from 1530 nm to 1565nm.

We claim:
 1. An optical coupler having an input and a plurality ofoutputs, the optical coupler comprising:a splitter portion having asplitter input for receiving an optical signal an plural outputsproviding respective fractions of the input optical signal therefrom andalso having an amplifier portion connected to supply optical signals tosaid splitter input upstream of the splitter portion, the amplifierportion including an optical amplifier for amplifying optical signalsreceived by the coupler input, the optical amplifier being provided withinput optical waveguiding means via which the optical amplifier isoptically pumpable by a remote pump laser, the amplifier having anoptical signal gain which is at least equal to the optical signal lossof the splitter portion; the optical amplifier being a doped fiberamplifier; the input optical waveguiding means being connected to thedoped fibre amplifier via a first WDM upstream of the doped fiberamplifier; the doped fibre amplifier being connected to the output via asecond WDM; and a filter downstream of the second WDM.
 2. An opticalcoupler having an input and a plurality of outputs, the optical couplercomprising:a splitter portion having a splitter input for receiving anoptical signal and plural outputs providing respective fractions of theinput optical signal therefrom and also having an amplifier portionconnected to supply optical signals to said splitter input upstream ofthe splitter portion, the amplifier portion including an opticalamplifier for amplifying optical signals received by the coupler input,the optical amplifier being provided with input optical waveguidingmeans via which the optical amplifier is optically pumpable by a remotepump laser, the amplifier having an optical signal gain which is atleast equal to the optical signal loss of the splitter portion; theoptical amplifier being a doped fiber amplifier; the input opticalwaveguiding means being connected to the doped fibre amplifier via afirst WDM; and the first WDM being downstream of the doped fibreamplifier.
 3. An optical system comprising an optical source, an opticalcoupler and a pump laser, the optical coupler being as claimed in claim2, the optical source being connected to the input of the opticalcoupler, and the pump laser being connected to the input optical waveguiding means.
 4. A system as claimed in claim 3, further comprising anagc unit, the agc unit and said pump laser being connected to the inputoptical waveguiding means by means of a further WDM.
 5. A system asclaimed in claim 4, wherein the input optical wave guiding means isconnected to the first WDM via another WDM.
 6. A system as claimed inclaim 5, wherein the downstream end of the doped fibre amplifier isconnected to said another WDM via a coupler.
 7. A system as claimed inclaim 6, wherein the coupler is a 10/90 coupler which directs 10% of theoutput of the doped fibre amplifier to said another WDM.
 8. A system asclaimed in claim 4, wherein the input is connected directly to theupstream end of the doped fibre amplifier.
 9. A system as claimed inclaim 3, wherein the optical source is a laser which emits light at 1536nm.
 10. A system as claimed in claim 3, wherein the optical source isconstituted by first and second lasers which are connected to the inputby a WDM and an optical wave guide.
 11. A system as in claim 10, whereinthe first laser emits light at 1300 nm, and the second laser emits lightat 1536 nm.
 12. A system as claimed in claim 11, wherein the first andsecond WDMs pass light at 1300 nm and at 1536 nm.
 13. A system asclaimed in claim 11, wherein means are provided for modulating aplurality of radio carrier signals with video signals, and means areprovided for mixing the modulated radio carriers, the resulting analoguesignal being used to modulate the second laser.
 14. A system as claimedin claim 3, wherein the pump laser emits light at 1480 nm.
 15. Asubstantially lossless optical signal splitter component for use in anoptical signal distribution network, said component comprising:anoptical information-bearing signal input port; an optical pump port; aplurality of optical information-bearing signal output ports; an opticalsignal amplifier coupled to amplify optical information-bearing signalsvia said input port using optical pump power from said optical pumpport; an optical signal splitter having an input port coupled to receiveamplified optical information-bearing signals from said amplifier and toprovide a respectively corresponding portion thereof to each of saidoutput ports; a source of optical pump power remotely located away fromsaid component and connected to said pump port via an optical fibrewaveguide separate from an optical fibre waveguide carrying opticalinformation-bearing signals to be distributed via said network; andmeans for returning a portion of an ac agc signal that has beenamplified by said amplifier in said component to said remotely locatedsource of optical pump power; and means located proximate said sourcefor adjusting the power of optical pump signals provided from saidsource to said pump port as a function of said returned ac agc signal.